In recent years, stretched films (so-called heat-shrinkable films) of polyvinyl chloride resins, polystyrene resins, polyester resins, and so forth have widely been used for uses as wrapping labels for protecting glass bottles and PET bottles and for indication purposes, cap seals, assembling packaging, and so forth. Among such heat-shrinkable films, polyvinyl chloride films have problems of low heat resistance, and generation of hydrogen chloride gas or dioxins at the time of incineration. Further, polystyrene films have problems of inferior solvent resistance, necessity of using special ink compositions at the time of printing, necessity of being incinerated at high temperature, and generation of a large quantity of black smoke accompanied with foul smell at the time of incineration. Heat-shrinkable films of polyesters having high heat resistance, that is easy to be incinerated and excellent in solvent resistance, therefore, tend to be used widely as shrinkable labels, and thus the use amount tends to be increased along with increase of the distribution amount of PET containers.
Those which can be shrunk greatly in the width direction have popularly been used as common heat-shrinkable polyester films. Such heat-shrinkable polyester films having a main shrinkage direction in the width direction are often stretched at a high ratio in the width direction to develop shrinkage characteristics in the width direction, but are subjected to stretching at only a low ratio in the lengthwise direction orthogonal to the main shrinkage direction, or sometimes not stretched. As described above, films which are subjected only to stretching at a low ratio in the lengthwise direction and films which are stretched only in the width direction have a defect that the films are inferior in the mechanical strength in the lengthwise direction.
In the case of being used as a label film for bottles or a banding film for bundling lunch boxes, films have to be heat-shrunk after circularly fit on a bottle or a lunch box. Therefore, when being fit on a bottle or a lunch box as a banding film, heat-shrinkable films which can be heat-shrunk in the width direction have to be formed in a circular body in a manner that the width direction is the circumferential direction, and then the circular body has to be cut at every prescribed length and fit on a bottle or a lunch box manually. Consequently, it is difficult for label films or banding films made of heat-shrinkable films which are heat-shrunk in the width direction to be fit on bottles or lunch boxes at a high speed. For this reason, those which have recently been desired are films which can be heat-shrunk in the lengthwise direction and which are therefore suitable for being directly wound around and fit on the circumference of a bottle or a lunch box from a film roll and then heat-shrunk in the lengthwise direction. There is no need to carry out a center sealing step of forming a film tubular body and sealing the tubular body or processing such as cutting and manual covering, and high speed fitting is made possible.
Further, from an environment aspect, films made from a recycled raw material of PET bottles are highly desired. Since common heat-shrinkable polyester films are produced by using a raw material containing a large quantity of an amorphous component for providing shrinkage characteristics, the ratio of the recycled raw material to be mixed is limited, and it is impossible to provide heat-shrinkable polyester films containing a large quantity of a recycled raw material.
For example, an ideal heat-shrinkable polyester film which has a main shrinkage direction in the vertical direction and high mechanical strength in vertical and transverse directions is known (see Patent Document 1). However, the film contains not less than 10 mol % of one or more kinds of monomer components, which are capable of serving as amorphous components, relative to the total of the polyester resin components, and the upper limit of addition ratio of the recycled raw material is thus spontaneously limited.
Further, in terms of the requirement for appearance of a film after shrinkage, it is required that distortion after shrinkage is slight. It is known that distortion after shrinkage is caused due to tilt of the main orientation direction of molecules of a film from the lengthwise direction or the width direction of the film. In terms of distortion, the tilt of orientation of molecules is ideally slight from the center part to the end part of a film.
Further, in terms of the appearance of a shrunk film, it is desired that the number of flaws on the film surface is small. A flaw on the film surface causes defects such as print omission at the time of printing on the film. Generally, at the time of heat-stretching a film, it is necessary for the film to be heated to a temperature not less than Tg, but a film at a temperature not less than Tg generates a flaw on the film surface due to friction by rolls of a film formation machine when brought into contact with the rolls or shift of the film. In the case where a common vertical stretching machine is used, a film is heated while being brought into contact with a roll at a temperature as high as a temperature not less than Tg, so that a flaw tends to be caused easily.
For example, in the case of Patent Document 1, many flaws are formed by bringing a film into contact with a plurality of rolls at a high temperature not less than Tg of the film in a vertical stretching machine to be used for film production, and it is therefore not preferable in terms of the appearance. Further, since neck-in occurs abruptly only in the peripheries of end parts, the difference of molecular orientation angle is significant in the peripheries of end parts, and the molecular orientation angle is wide. Therefore, when the film portions near the end parts are shrunk, distortions are caused, and it is not preferable in terms of the appearance.